If you're using Photoshop(Most art programs have this tool too..) then I would use the lasso tool next time around the full image, or just go back and fill in the black around the characters 'cause having the white around the edges like that makes it look unfinished..

the white background was a contrast with the actual art more than the black on considering the characters are both in darker colors but the cotton parts of her clothing would had just looked like it was uncolored and wouldn't stand out as well